Referring to FIG. 1, a typical rotor 100 of a synchronous reluctance machine consists of a plurality of rotor disks 110 stacked together in axial direction. Each rotor disk 110 comprises essentially a disk body of high magnetic permeability material, and longitudinal flux barriers 120 of low magnetic permeability material. Typically the flux barriers 120 are created by cutting material from the disk body, the low magnetic permeability material thereby being air. The flux barriers 120 are configured to give the rotor disk 110 an anisotropic magnetic structure such that axes of maximum reluctance i.e. q-axes 130, and axes of minimum reluctance i.e. d-axes 140 are formed. Each pole of the rotor disk 110 typically comprises 3-5 radial distanced longitudinal flux barriers 120 in turns with flux paths 150 of corresponding shape. A radial extending symmetry line of each pole coincides with a q-axis 130. The rotor disk 110 is mechanically self-sustained in that the flux paths 150 are connected to one another by narrow tangential ribs 160 at a disk periphery 170, and eventually also with radial bridges 180 at q-axes 130.
Between neighbouring rotor poles there are spokes 190 extending in radial direction between a shaft opening 200 and the disk periphery 170. Typically, if the spokes 190 are symmetrical, the symmetry axis of each spoke 190 coincides with a d-axis 140. The spokes 190 are typically solid elements consisting of the disk body material, but they may comprise some holes or openings in different shapes and for different purposes such as for inserting tie bolts or for functioning as a flux barrier 120.
JP 2004-254354 discloses a rotor disk with triangle-shaped openings in the spokes. These openings function as flux barriers.
JP 2006-042467 discloses a rotor disk with wide openings in the spokes. The function of the openings is to affect the rigidity of the rotor disc such that it can better stand deformations due to the centrifugal force.
JP 2001-136717 discloses a rotor disk with relatively large openings in the spokes. JP2001-136717 does not explain the purpose of the openings, but the explanation is probably related to the mechanical properties of the rotor disk. The width of the openings at the radial inward ends of the same is only slightly greater than the overall width of the openings.
U.S. Pat. No. 7,560,846 discloses in FIG. 13 a rotor disk comprising triangular shaped openings in the spokes. The openings are configured to receive a coupling member fixing and orienting the rotor disks together.
U.S. Pat. No. 6,300,703 discloses in FIG. 31 a rotor disk with a long and narrow opening in the spokes. U.S. Pat. No. 6,300,703 does not explain the purpose of the openings, but they appear to function as additional flux barriers.
When a synchronous reluctance machine operates, iron losses in the rotor 100 cause the rotor 100 to heat up. Although such losses are relatively low in a synchronous reluctance machine, the temperature at the flux paths 150 separated by flux barriers 120 still may become quite high because the generated heat cannot be effectively conducted away. It is only the radial innermost flux paths 150, i.e. the spokes 190, that have a large heat conducting area towards the rotor shaft. All the remaining flux paths 150 are connected to the rotor shaft only via narrow ribs 160 or bridges 180 which do not provide an adequate heat conducting capacity for keeping the rotor temperatures down. Therefore, in a conventional synchronous reluctance machine there is a great temperature difference between the spokes 190 and the remaining flux paths 150 during a long-term operation.
Great temperature differences between different parts of a rotor disk 110 cause thermal tensions within the same. Together with centrifugal load these tensions result in an excess deformation of the rotor disk 110 which lead to hairline cracks and ultimately destroy the rotor disk 110. The prior art rotor disks 110 do not provide a satisfactory solution for preventing this from happening. FIG. 2 shows a simulated detail of a conventional rotor disk 110 in a deformed state under a certain load condition. The deformations in FIG. 2 are exaggerated for the sake of illustration.